This invention relates generally to improvements in bone grafts such as spinal fusion cages of the type designed for human implantation between adjacent spinal vertebrae, to maintain the vertebrae in substantially fixed spaced relation while promoting interbody bone ingrowth and fusion therebetween. More particularly, this invention relates to an implantable bone graft such as a spinal fusion cage having an improved combination of enhanced mechanical strength together with osteoinductive and osteoconductive properties, in a ceramic-based device that additionally and beneficially provides radiolucency for facilitated post-operative monitoring.
Implantable interbody bone grafts such as spinal fusion devices are known in the art and are routinely used by spine surgeons to keep adjacent vertebrae in a desired spaced-apart relation while interbody bone ingrowth and fusion takes place. Such spinal fusion devices are also used to provide weight bearing support between adjacent vertebral bodies and thus correct clinical problems. Such spinal fusion devices are indicated for medical treatment of degenerative disc disease, discogenic low back pain and spondylolisthesis. These conditions have been treated by using constructs, typically made from metals such as titanium or cobalt chrome alloys such as used in orthopedic implants, and allograft (donor) or autograft (patient) bone to promote bone ingrowth and fusion.
Typical interbody spinal fusion devices, such as plugs for example, have hollow or open spaces that are usually filled with bone graft material, either autogenous bone material provided by the patient or allogenous bone material provided by a third party donor. These devices also have lateral slots or openings which are primarily used to promote ingrowth of blood supply and grow active and live bone. These implants may also have a patterned exterior surface such as a ribbed or serrated surface or a screw thread to achieve enhanced mechanical interlock between adjacent vertebrae, with minimal risk of implant dislodgement from the site. See, for example, U.S. Pat. Nos. 5,785,710; and 5,702,453. Typical materials of construction for such interbody spinal fusion devices include bio-compatible carbon fiber reinforced polymers, cobalt chrome alloys, and stainless steels or titanium alloys. See, for example, U.S. Pat. No. 5,425,772.
Most state-of-the-art spinal fusion implants are made from titanium alloy and allograft (donor) bone, and have enjoyed clinical success as well as rapid and widespread use due to improved patient outcomes. However, titanium-based implant devices exhibit poor radiolucency characteristics, presenting difficulties in post-operative monitoring and evaluation of the fusion process due to the radio-shadow produced by the non-lucent metal. There is also clinical evidence of bone subsidence and collapse which is believed to be attributable to mechanical incompatibility between natural bone and the metal implant material. Moreover, titanium-based implant devices are primarily load bearing but are not osteoconductive, i.e., not conducive to direct and strong mechanical attachment to patient bone tissue, leading to potential graft necrosis, poor fusion and stability. By contrast, allograft bone implants exhibit good osteoconductive properties, but can subside over time as they assimilate into natural bone. Further, they suffer from poor pull out strength resulting in poor stability, primarily due to the limited options in machining the contact surfaces. Allograft bone implants also have variable materials properties and, perhaps most important of all, are in very limited supply. A small but finite risk of disease transmission with allograft bone is a factor as well. In response to these problems some developers are attempting to use porous tantalum-based metal constructs, but these have met with limited success owing to the poor elastic modulii of porous metals.
A typical titanium alloy spinal fusion device is constructed from a hollow cylindrical and externally threaded metal cage-like construct with fenestrations that allow communication of the cancellous host tissue with the hollow core, which is packed with morselized bone graft material. This design, constrained by the materials properties of titanium alloys, relies on bony ingrowth into the fenestrations induced by the bone graft material. However, the titanium-based structure can form a thin fibrous layer at the bone/metal interface, which degrades bone attachment to the metal. In addition, the hollow core into which the graft material is packed may have sub-optimal stress transmission and vascularization, thus eventually leading to failure to incorporate the graft. Mechanical stability, transmission of fluid stress, and the presence of osteoinductive agents are required to stimulate the ingrowth of vascular buds and proliferate mesenchymal cells from the cancellous host tissue into the graft material. However, most titanium-based spinal fusion devices in use today have end caps or lateral solid walls to prevent egress of the graft outwardly from the core and ingress of remnant disc tissue and fibroblasts into the core.
Autologous (patient) bone fusion has been used in the past and has a theoretically ideal mix of osteoconductive and osteoinductive properties. However, supply of autologous bone material is limited and significant complications are known to occur from bone harvesting. Moreover, the costs associated with harvesting autograft bone material are high, requiring two separate incisions, with the patient having to undergo more pain and recuperation due to the harvesting and implantation processes. Additionally, autologous cancellous bone material has inadequate mechanical strength to support intervertebral forces by itself, whereby the bone material is normally incorporated with a metal-based construct.
Ceramic materials provide potential alternative structures for use in spinal fusion implant devices. In this regard, monolithic ceramic constructs have been proposed, formed from conventional materials such as hydroxyapatitie (HAP) and/or tricalcium phosphate (TCP). See, for example, U.S. Pat. No. 6,037,519. However, while these ceramic materials may provide satisfactory osteoconductive and osteoinductive properties, they have not provided the mechanical strength necessary for the implant.
Thus, a significant need exists for further improvements in and to the design of bone grafts such as spinal fusion implant devices, particularly to provide a high strength implant having high bone ingrowth and fusion characteristics, together with substantial radiolucency for effective and facilitated post-operative monitoring.
Hence, it is an object of the present invention to provide an improved bone graft such as an interbody spinal fusion implant or cage made from a bio-compatible open pore structure, which has a radiolucency similar to that of the surrounding bone. It is also an object of the present invention to provide a substrate of high bio-mechanical strength for carrying biological agents which promote inter-vertebral bone ingrowth, healing and fusion. It is a further objective of the present invention to provide an interbody fusion device which has mechanical properties that substantially match that of natural bone, by using ceramic construct materials rather than metal.
In accordance with the invention, an improved bone graft such as a spinal fusion cage is provided for human implantation into the space between a pair of adjacent vertebrae, following removal of disc material between endplates of the adjacent vertebrae, to maintain the adjacent vertebrae in a predetermined and substantially fixed spaced relation while promoting interbody bone ingrowth and fusion. In this regard, the improved spinal fusion cage of the present invention is designed for use in addressing clinical problems indicated by medical treatment of degenerative disc disease, discogenic lower back pain, and spondylolisthesis.
The improved bone graft, as embodied in the form of the improved spinal fusion cage, comprises a porous or open-celled substrate block formed from a ceramic composition having a relatively high bio-mechanical strength and load bearing capacity. A preferred composition for the ceramic substrate block comprises a two phase alumina-zirconia composition having a suitable size and shape for seated implantation into the inter-vertebral space, with generally planar or appropriately shaped faces seated respectively upon the endplates of the adjacent vertebrae. In a preferred form, the substrate block comprises an alumina-zirconia composition with an alumina to zirconia ratio ranging from about 100% (i.e., 2:1) to about 25% (i.e., 1:4). More preferably, the substrate block composition comprises about 10% to about 20% by volume zirconia in alumina. Either yttria stabilized zirconia (about 2.5 to about 5 mol % yttria in zirconia) or ceria stabilized zirconia (about 2.5 to about 15 mol % ceria in zirconia) is preferably used for the zirconia phase. The substrate block is porous, having a porosity of about 10% to about 50% by volume with open pores distributed throughout and a pore size range of from about 5 to about 500 microns. In a preferred form, the porosity of the substrate block is gradated from a first relatively low porosity region emulating or mimicking the porosity of cortical bone to a second relatively higher porosity region emulating or mimicking the porosity of cancellous bone.
The porous substrate block is internally and externally coated with a bio-active surface coating material selected for relatively high osteoconductive and osteoinductive properties, such as a hydroxyapatite or a calcium phosphate material.
The thus-formed bone graft can be made in a variety of shapes and sizes to suit different specific implantation requirements. Preferred shapes include cylindrical or partly cylindrical with substantially flat opposite ends and a tapered or lordotic cross section to suit the required curvature of the inter-vertebral space, in the case of a spinal fusion device. The exterior surface of the cylindrical body may include threads or ribs for facilitated and secure screw-in placement, for example, between adjacent vertebrae. Alternative preferred shapes include a generally rectangular block which may also include serrations or the like on one or more exterior faces thereof, and/or may have a tapered or lordotic cross section for improved fit into the inter-vertebral space. The bone graft may desirably include a posterior end having an engagement means such as a threaded socket for releasible engagement with a suitable insertion tool. In addition, the bone graft may also include one or more laterally open recesses or bores for receiving and supporting osteoconductive bone graft material, such as allograft (donor) or autograft (patient) material.
Further alternative bone graft configurations may include controlled porosity gradations to define a relatively low porosity first region or regions substantially emulating cortical bone, to define a high strength loading bearing zone or strut for absorbing impaction and insertion load, in combination with one or more relatively high porosity second regions substantially emulating cancellous bone for contacting adjacent patient bone for enhanced bone ingrowth and fusion.
The resultant bone graft exhibits relatively high mechanical strength for load bearing support, for example, between adjacent vertebrae in the case of a spinal fusion cage, while additionally and desirably providing high osteoconductive and osteoinductive properties to achieve enhanced bone ingrowth and interbody fusion. Importantly, these desirable characteristics are achieved in a structure which is substantially radiolucent so that the implant does not interfere with post-operative radiographic monitoring of the fusion process.
In accordance with a further aspect of the invention, the bone graft may additionally carry one or more therapeutic agents for achieving further enhanced bone fusion and ingrowth. Such therapeutic agents may include natural or synthetic therapeutic agents such as bone morphogenic proteins (BMPs), growth factors, bone marrow aspirate, stem cells, progenitor cells, antibiotics, or other osteoconductive, osteoinductive, osteogenic, or any other fusion enhancing material or beneficial therapeutic agent.
Other features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.